Magnet structure for nuclear magnetic resonance imaging apparatus

ABSTRACT

A magnet structure for a Nuclear Magnetic Resonance imaging apparatus includes at least two opposing magnetic pole pieces, which are located at a certain distance from each other and delimit an imaging region. The pole pieces are formed by at least one massive layer of a magnetically permeable material, and at least one layer of magnetically permeable material having a pack of superimposed sheets or foils, electrically insulated from each other. Each of the sheets has cuts arranged over the surface of the sheet in positions that are at least partly non coincident with the cuts of at least one, or both adjacent sheets. The magnetically permeable sheets or foils have a first face and a second face and the cuts are so arranged on each sheet that the cuts of a sheet or foil are offset and not coincident with respect to the cuts of an adjacent sheet or foil, when said adjacent sheet is laid over the previous sheet in an overturned position, i.e. with the first face turned toward the first face of the first sheet or with the second face of said adjacent sheet turned toward the second face of the first sheet.

FIELD OF THE INVENTION

[0001] The invention relates to a magnet structure for a NuclearMagnetic Resonance imaging apparatus and the fabrication thereof. Moreparticularly the invention relates to a magnet structure having at leasttwo opposed pole pieces, which are located at a certain distance fromeach other and delimit an imaging region, wherein the pole pieces areformed by at least one massive layer of a magnetically permeablematerial, and at least one layer of magnetically permeable materialcomprising a pack of superimposed sheets or foils, electricallyinsulated from each other, each sheet having cuts arranged over thesurface of the sheet in positions that are at least partly noncoincident with the cuts of at least one, or both adjacent sheets.

BACKGROUND OF THE INVENTION

[0002] Several different types of magnet structures are known that havethe above construction. U.S. Pat. No. 5,555,251 discloses a magnetstructure which has two pole pieces having a massive ferromagnetic layerand a layer made of laminated ferromagnetic foils or sheets. In thisarrangement, sheets are made of one piece and have a number of radialcuts, arranged from a center of the sheet. The sheets have a circularshape, coaxial with the center around which the cuts are radiallyarranged. All the sheets have the same shape and the same pattern ofcuts.

[0003] The cuts of each sheet are offset with respect to an adjacentsheet by rotating each sheet relative to the adjacent sheet by an anglesmaller than the angular distance between two successive radial cuts.

[0004] As a result, in order to form the laminated layer of each polepiece, the sheets must be properly offset before being bonded togetherby an adhesive layer or an electric insulating and adhesive layer whichcoats the sheets. While the assembly of the laminated layer isintrinsically simple, it is still dependent on the position of thesheets relative to each other, and this is a parameter to be accountedfor during manufacture of the pole pieces. Moreover, this may generateerrors in the angular positioning of the sheets.

[0005] Also, due to the one-piece construction of the sheets, handlingproblems may arise when the sheets, as well as the pole pieces have arelatively large size. An additional problem consists in that that thesheets that require a high dimensional precision and are made of aspecial magnetically permeable material are generally not fabricated ofsufficiently large sizes, and any size increase thereof would involve aconsiderable cost increase. In this case, the provision of one-piecesheets is not feasible or involves higher costs for the magnetstructure.

[0006] The invention is based on the problem of providing a magnetstructure as described hereinbefore which, thanks to simple andinexpensive arrangements, allows easier handling, particularly for thefabrication of large-sized magnets, and helps to obviate the abovedrawbacks.

OBJECTS AND SUMMARY

[0007] An embodiment of the invention solves the above problems byproviding a magnet structure as described hereinbefore, wherein themagnetically permeable sheets or foils have a first face and a secondface and the cuts have such a width and such a length and are soarranged on each sheet, that the cuts of a sheet or foil are offset andnot coincident with respect to the cuts of the adjacent sheet or foil,when said adjacent sheet lies over the previous sheet in an overturnedposition, i.e. with the first face turned toward the first face of thefirst sheet or with the second face of said adjacent sheet turned towardthe second face of the first sheet.

[0008] In accordance with an embodiment of the invention, each sheet maybe divided into two halves along an axis parallel to a sheet overturningaxis, about which each successive sheet is overturned by 180° relativeto the adjacent preceding sheet of the layer of sheets of each polepiece. In this embodiment, an identical cut pattern may be provided forall sheets, which pattern differs in cut arrangement, orientation,length, and/or width in the two halves of each sheet such that, when twosheets are superimposed in a mutually overturned or reversed condition,i.e. with the first faces or the second faces of said two sheets facingtoward each other, the cuts of a first sheet are disposed in offsetpositions with respect to the cuts of the second sheet in both halves ofthe adjacent sheets.

[0009] As a result, identical sheets may be provided for each layer, andeach superimposed sheet needs simply be overturned with respect to theunderlying sheet of a pack of sheets that forms the non massive layer ofa pole piece of the magnet structure.

[0010] Particular advantageous embodiments will be described in greaterdetail in the following description and will form the subject ofspecific subclaims.

[0011] Cuts may be arranged according to a few rules that simplify boththe cut pattern design and the actual cut forming process.

[0012] A first rule may consist in disposing cuts along parallel axes,forming a set of parallel axes, the sets of axes on the first and secondhalves of the sheet being oriented parallel to each other and to anoverturning axis, and there being provided a distance of the first axisof each set of axes from the center axis of the sheet, which is parallelto or coincident with the overturning axis, said distance beingdifferent for the sets of axes on the first half and on the second halfof the sheet respectively.

[0013] Cuts may be continuous or discontinuous along the correspondingpositioning axis of the set of parallel positioning axes. Discontinuouscuts form whole portions or bridges of sheet material along cutpositioning axes.

[0014] According to a variant, as shown in FIGS. 1 to 3, the sets of cutpositioning axes on the two sheet halves have an inclined, symmetricallydivergent or convergent orientation, for the first and second sheethalves, with respect to the center axis of the sheet, which is parallelto or coincident with the sheet overturning axis, the intersectionpoints of the set of parallel arrangement axes on the first sheet halfwith said center axis being provided in intermediate positions betweenthe intersection points of the set of parallel positioning axes of thesecond sheet half. Also, the intersection points of the two sets ofparallel cut (2, 2′) positioning axis on the first and second halves(202, 302) of the sheet (2, 2′) may be interleaved and equally spacedalong the center axis, which is parallel to or coincident with theoverturning axis.

[0015] According to another variant, the sets of cut positioning axes onthe two sheet halves have an inclined orientation with respect to thecenter axis of the sheet, which is parallel to or coincident with thesheet overturning axis, the intersection points between the set ofparallel positioning axes on the first half of the sheet and said centeraxis being situated in intermediate positions with respect to theintersection points between the positioning axes of the set of parallelpositioning axis of the second sheet half and said center axis, and eachcut along each positioning axis being discontinuous and forming anunbroken sheet portion, the succession of the unbroken sheet portionsand of the cut parts along the positioning axes being inverted from thefirst half to the second half of the sheet, whereas the pitch of thecuts and unbroken portions along each arrangement axis is such that,when the first sheet half is overturned against the second sheet half,the cuts along each positioning axis of the first sheet half intersectthe cuts along each positioning axis of the second sheet half atunbroken portions, and vice versa.

[0016] No restriction is provided to patterns and shapes of cuts. Itshall be further noticed that the pattern is only conceived on one ofthe ideal halves of the sheet, this pattern defining the cut-freeportions of the same half, hence the position of cuts in the secondhalf, considering that, by overturning the sheet, pattern orientationwill be symmetrically reversed with respect to the overturning axisand/or to the center axis that ideally divides the sheet in halves.

[0017] The above arrangement allows to conceive cut patterns in whichthe direction and arrangement of cuts varies as a function of theorientation of the turns or conductors of one or more possible gradientcoils associated to the magnet structure, as is typical in NuclearMagnetic Resonance imaging apparatus.

[0018] When cut patterns on the sheets provide a repeated, orderly andrecurring cut arrangement, cuts may be simply formed on the two idealhalves of the sheet, by suitably offsetting the pattern used on oneideal sheet half, in one or two directions subtending the plane thatcontains the sheet and/or also in a possible direction of rotation orcombination of said displacements, with respect to the second sheethalf, such that, when said second half is overturned and laid over thefirst ideal half of an underlying sheet, the cuts of the second idealhalf coincide with the cut-free portions of said first ideal half of theunderlying sheet and vice versa.

[0019] One variant of the cut arrangement pattern on the sheets consistsin a radial arrangement thereof from a center, an identical angulardistance being provided between individual radial lines along which cutsare provided and the cuts forming a half ring of cuts on each idealsheet half, whereas the half ring on the second ideal sheet half isrotated with respect to the half ring on the first ideal sheet half tosuch an extent that when two adjacent sheets are in the superimposed,overturned condition, the cuts of the two ideal halves of a sheet aredisposed coincident with the radial intermediate portions between thecuts of the two ideal halves of the adjacent sheet.

[0020] According to an additional variant embodiment, each ideal sheethalf is divided into a plurality of regions by sets of crossed cuts,i.e. sets of cuts having different orientations, which are broken atcrossing areas to form bridges of material that connect together thesheet regions, the crossed sets of parallel cuts on the second idealsheet half being offset, relative to said second half with respect tothe arrangement in the first ideal sheet half, and to such an extent andin such directions that the bridges of material that connect thedifferent areas fall within regions of an adjacent sheet.

[0021] Such cut pattern is derived from and detailed in patentapplication SV2001A00009, published by the owner hereof.

[0022] According to another embodiment of the invention, when the sheetmaterial is not available in sizes large enough to form each sheet fromone piece or when the sheet has a too large size to allow a sufficientlyeasy handling thereof in the one-piece construction, the inventionprovides that each sheet be formed by at least two adjacent sheet parts,separated by a parting line, said parting line being provided ineccentric position and/or orientation with respect to the separationaxis between the two sheet halves, such that the parting lines betweenthe parts of two superimposed adjacent sheets do not coincide.

[0023] The parting line between the two sheet parts extends in acut-free portion, so that said parting line does not intersect any cuton the sheet and/or possibly on one or two adjacent sheets.

[0024] Obviously, such arrangement is not limited to the manufacture ofsheets that are only formed by two adjacent parts, but may be alsoextended to sheets formed by three or more adjacent parts.

[0025] Thanks to the above arrangements, by using a single sheet type,either having a one-piece construction or formed by two or more adjacentparts, a simple alternate, overturned arrangement of adjacent sheets inthe pack, provides a magnetically permeable layer formed by a pack ofsheets or foils, which has cuts in offset positions from one sheet tothe other in the pack, and such as to effectively suppress eddy currentsproduced in the pole pieces.

[0026] During the assembly, there is no need to account for a preciseoffset size between superimposed sheets, as the sheet simply needs to beoverturned or reversed each time before disposing it on the sheet pack.Such operation is considerably simplified by the fact that the sheets ofa pack generally have substantially or precisely the same plan size,whereby the sheets may be aligned in the proper position by simplyaligning the peripheral edges thereof.

[0027] Fabrication of sheets or foils is also considerably simplified byproviding a repetitive cut pattern on each half. Hence, after formingthe cuts of the first half, the second half may be treated in much thesame manner, by simply offsetting the second half along a predeterminedpath of the cutting tools used to form the cuts in said first half. Therotational and/or translational offset in one and/or two perpendiculardirections, subtending a plane parallel to the sheet surface isdetermined according to the above description regarding the arrangementof cuts in the two sheet halves. This considerably simplifies sheetfabrication.

[0028] Regarding the parting lines or cuts, these are simply made bytrimming adjacent edges of two adjacent sheet parts.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0029] The characteristics of the invention and the advantages derivedthere from will appear more clear from the following description of nonlimiting embodiments, illustrated in the annexed drawings, in which:

[0030]FIG. 1 shows a plan view of a first sheet for use in a magnetstructure according to a first embodiment of the invention.

[0031]FIG. 2 shows a plan view of a second sheet in the overturnedposition for use in a magnet structure according to a first embodimentof the invention.

[0032]FIG. 3 shows the first and second sheets of FIGS. 1 and 2 in asuperimposed position according to a first embodiment of the invention.

[0033]FIG. 4 shows a plan view of a first sheet comprising analternative pattern of cuts.

[0034]FIG. 5 show a plan view of a second sheet in the overturnedposition.

[0035]FIG. 6 shows the first and second sheets of FIGS. 4 and 5 in asuperimposed position.

[0036]FIG. 7 shows a plan view of a first sheet comprising an anotheralternative pattern of cuts.

[0037]FIG. 8 shows a plan view of a second sheet in the overturnedposition.

[0038]FIG. 9 shows the first and second sheets of FIGS. 7 and 8 in asuperimposed position.

[0039]FIG. 10 shows a plan view of a first sheet comprising an stillanother alternative pattern of cuts.

[0040]FIG. 11 shows a plan view of a second sheet in the overturnedposition.

[0041]FIG. 12 shows the first and second sheets of FIGS. 10 and 11 in asuperimposed position.

[0042]FIG. 13 shows a plan view of a first sheet comprising a radialpattern of cuts.

[0043]FIG. 14 shows a plan view of a second sheet in the overturnedposition.

[0044]FIG. 15 shows the first and second sheets of FIGS. 13 and 14 in asuperimposed position.

[0045]FIG. 16 shows a sheet formed by two or more separate adjacentparts in accordance with a second embodiment of the invention.

[0046]FIG. 17 shows a sheet formed by two or more separate adjacentparts with an alternative parting line between the parts.

[0047]FIG. 18 shows a sheet formed by two or more separate adjacentparts with another alternative parting line.

[0048]FIG. 19 shows a cross section of a magnet structure according toan embodiment of the invention.

[0049]FIG. 20 shows a detail of a section of the layer formed by thesheets of the magnet structure pole piece.

[0050] FIGS. 21 to 25 show different patterns of the line that separatesa sheet into two parts according to the second embodiment of theinvention.

[0051]FIG. 26 shows a plan view of a first sheet for use in a magnetstructure according to a third embodiment of the invention.

[0052]FIG. 27 shows a plan view of a second sheet for use in a magnetstructure according to a first embodiment of the invention.

[0053]FIG. 28 shows the first and second sheets of FIGS. 26 and 27 in asuperimposed position according to a third embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0054] Referring to FIG. 19, a magnet structure of a Nuclear MagneticResonance imaging apparatus has two opposing pole pieces 1, betweenwhich a static magnetic field is generated. Each pole piece is composedof magnetic field generating means, here a layer of magnetized material101, a magnetically permeable layer laying over the face thereof turnedtoward the opposite pole piece, e.g. a ferromagnetic layer, or the like.

[0055] Said layer is in turn composed of a massive layer, denoted 201,an additional layer 301 further laying over it, which layer consists ofsuperimposed magnetically permeable sheets or foils 1, which form a packof sheets, pressed tightly together.

[0056] The pole pieces are supported and/or enclosed by a structure,also made of a magnetically permeable material, which has the additionalfunction of closing the magnetic field between the pole pieces, whichstructure or yoke is generally denoted 3. The structure that is shown inFIG. 19 is a typical structure having an inverted U or C shape. However,the architecture of the structure is not relevant for the purpose ofthis invention, which applies to the pole pieces of any magnetstructure.

[0057] Within the permeable material of the pole pieces, eddy currentsare induced, which are preferably suppressed or drastically reduced,whereby the layer 301 is formed by alternate superimposed sheets 2, 2′,each having a number of cuts 102, 102′, arranged in various patternsover the sheet surface.

[0058] Nevertheless, advantages are obtained if cuts 102, 102′ are notin coincident positions within the pack of superimposed sheets 2, 2′.

[0059] According to a first embodiment, as shown in FIGS. 1 to 3, theinvention provides identical sheets, each having different arrangementpatterns on the two halves 202, 302 of its surface. The pattern of cutson the two halves are arranged such that, by laying one sheet 2 of thepack of sheets over an identical sheet, after overturning it, as if byleafing through a book, the cuts 102, 102′ of the sheets 2, 2′ that formthe pack of sheets are not coincident, except for one possible,tolerable intersection point.

[0060] In practice, if the two faces of a sheet are defined as a firstface or front face and second face, or rear face, in the succession ofsuperposed sheets which form the pack, underlying sheets do not adhereby their front face against the rear face of the sheet lying over them,but are overturned, with the front face turned toward the front face ofthe underlying sheet and adhering against it.

[0061]FIG. 1 shows a cut arrangement pattern according to a firstembodiment of the invention. The cuts 102 on one of the two halves 202of the sheet 2 are oriented to form a set of equally spaced, parallelaxes, having a predetermined inclination with respect to a generaloverturning axis of the sheet 2, here conveniently shown coincident withthe center axis. On the other half 302 of the sheet 2, the pattern ofcuts 102′ is not only rotationally symmetric with regard to theinclination of the set of parallel axes, but the set of parallel axes isoffset along the central overturning axis to a predetermined extent,which may be equal to half the distance between two parallel axes.

[0062] Therefore, the pack is formed as follows:

[0063] Over the sheet 2 of FIG. 1, an additional identical sheet is laidin an overturned position, which sheet is shown in FIG. 2 and is denoted2′, with the cuts thereon being denoted 102′.

[0064] The cut arrangement resulting from such superimposition is shownin FIG. 3. Obviously, the cuts 102, 102′ of the underlying sheet are notvisible in actual conditions.

[0065] The pack of sheets that forms the layer 301 is formed by analternation of sheets 2, having a position like that shown in FIG. 1 andsheets 2′, having the overturned position as shown in FIG. 2.

[0066] As is apparent from the above description, the pack of sheets isthereby easily formed. In fact, the pack is formed by superimposingseveral sheets 2, 2′ having the same pattern of cuts 102, alternately inthe two positions described above, i.e. with the front face turnedtoward the pack and with the front face turned in the direction oppositeto the pack.

[0067] As is clearly shown in FIG. 20, which is a simplified andenlarged sectional view of a pack of sheets 2, 2′, the cuts 102, 102′are disposed on each sheet in an offset position with respect to thoseof the preceding sheet. Any sheet laid over the pack will have cuts 102offset with respect to those of the directly underlying sheet 2′ butcoincident with those of the sheet 2 underlying said directly underlyingsheet in the pack of sheets.

[0068] FIGS. 4 to 6 show an alternative cut pattern different from theembodiment of FIGS. 1 to 3. In this case, the cuts 102 and the cuts 102′are oriented in two transverse, non parallel directions.

[0069] The cuts 102 of the sheets may have a limited and predeterminedlength, there being provided, like in the pattern of FIGS. 1 to 3, a rowof cuts aligned along one or more or all axes of the set of parallelaxes, which defines the cut arrangement. Hence, the cuts have a limitedlength and are separated by cut-free regions 402.

[0070] The cut pattern selected for the two halves of the sheet 2 mayalso include cut-free regions and, besides combining the cut arrangementpatterns, it may include offset cut-free regions for the cuts of twoadjacent sheets 2, like in the embodiment of FIGS. 1 to 3.

[0071] According to a first method for fabricating the pack 301 ofsheets, the sheets 2, 2′ are coated with a layer of insulating varnish,denoted 4 [NOT CURRENTLY SHOWN IN FIGURES}, which additionally has thefunction of an adhesive to bond together the sheets 2, 2′ of the pack.Therefore, the sheets 2, 2′ are varnished before being superimposed toform the pack as described above, and then the pack is hot or coldpressed. The sheets may advantageously have centering or alignment holesfor the insertion of centering or alignment pins (not shown in detail).The pack of sheets may be formed separately and fitted onto the massivelayer 201 of the pole pieces 1 or the pack may be formed directly on themassive layer 201 of the pole pieces 1, which has, in this case, asupporting function and may have centering and/or alignment pins orcentering and alignment holes for the insertion of said pins.

[0072] According to another method for fabricating the pack 301 ofsheets, a layer of glue is used instead of the varnish. The layer ofglue may be provided as adhesive films or may be applied, for instancespread, sprayed or the like, on the face of at least one sheet (2, 2′).After superimposing the sheets, the latter may be hot or cold pressed.

[0073] According to yet another method of fabricating the pack ofsheets, a thermosetting adhesive layer is disposed between the sheets.This layer may be provided as a film, or applied by spreading, brushing,spraying, etc. Once the pack of sheets is formed with interposedthermosetting adhesive layers, the pack is hot and vacuum pressed toactivate the thermosetting adhesive.

[0074] As a thermosetting adhesive, advantages are obtained by usingfoils of a material known as vetronite, which is widely used in themanufacture of printed circuit headers.

[0075] A suitable material is, for instance, the material named PRGEP84,type 1080, sold by DRITRON SPA, whose characteristics are listed inwww.ditronlaminati.com.

[0076] In the embodiment of FIGS. 4 to 6, unlike the one of FIGS. 1 to3, the cut-free regions 102 which separate the cuts aligned on the sameaxis are coincident with the cuts 102′ of the adjacent sheet 2, and thecuts 102 of the underlying sheet never intersect the cuts 102′ of thedirectly adjacent, underlying sheet. In this embodiment, the cuts 102 oftwo adjacent sheets are oriented in transverse, particularly orthogonaldirections.

[0077] FIGS. 7 to 12 show, like the previous embodiments, two additionalcut patterns for use on the sheets according to the invention.

[0078] Like in the previous embodiments, the sheets are divided into twohalves, preferably symmetrically to a central overturning axis, thepatterns of cuts 102 being formed or simply positioned in differentmanners on the two halves so that cuts are offset when the pack ofsheets is formed, by placing the sheets alternately in one position andin an overturned position according to said axis.

[0079] In the cut patterns shown in FIGS. 7 and 8, the regions 202 and302 of the sheets 2, 2′ are divided by cuts oriented along two sets ofparallel axes, the directions of the axes of the two sets beingtransverse, particularly perpendicular. The cuts are not continuous butbroken at intersections. Hence, the sheet is divided by a grid of cuts102 into a plurality of adjacent polygonal, particularly squareportions, separated by the cuts and joined together by bridges ofmaterial at the corners of squares, as designated by 402.

[0080] In the embodiment of FIGS. 7 to 9, the difference between the cutarrangements of the two halves 202 and 302 of the sheets is obtained byoffsetting the position of the grid of cuts 102 to a predeterminedextent along both directions of the sets of cut positioning axes.

[0081] As is apparent from FIG. 9, which shows two superimposed sheets2,2′, according to the above embodiment, as well as the position andorientation of the cuts 102′ of the underlying sheet which are notactually visible, cuts intersect in this pattern.

[0082] If desired, the cuts may be also completed at corners, where thebridges of materials 402 are provided. If these bridges are sufficientlysmall, they may be cut off even with the pack of sheets in the assembledcondition. This operation, that may be performed, for instance, by lasercutting, causes the full separation of the polygonal portions. However,the hole that cuts off the bridges of material between polygonalportions in one sheet, produces a hole in the material of the polygonalportion coincident with the bridge of material 402. This hole causes nooperation problem.

[0083] The embodiment of FIGS. 10 to 12 differs from the previousembodiment in that the pattern on the portion 302 of the sheet 2 is notobtained by simply translating the pattern of the portion 202 in twoperpendicular directions. So, no grid is formed like in the portion 302,but the cuts, having different, particularly perpendicular directions,are disposed in two perpendicular directions and in such positions as tointersect.

[0084] The final pattern of cuts in the superimposed condition is notdifferent from the one of the embodiment as shown in FIGS. 7 to 9,however the cut arrangement is different at least for the portion 302 ofthe sheet.

[0085] FIGS. 13 to 15 show a further embodiment, wherein the cuts 102extend radially from the center of the plan view of the sheets 2.

[0086] For the sake of simplicity, FIG. 13 only shows one order ofradial and continuous cuts 102. Nevertheless, in order to improve thesheet strength, each cut 102 may be formed discontinuously, i.e.comprising a row of limited-length cuts, separated by bridges ofmaterial, along each radius.

[0087] Two radial sets of cuts may be also provided, a first setextending, with continuous or discontinuous cuts, to a certain distancefrom the periphery, and a second radial set, angularly interleaved withthe former, forming a ring of peripheral cuts, which extend from the endof the first set of cuts to the peripheral portion of the sheet. Theradial cuts of the peripheral set may be also arranged to start closerto the center as compared with the outer radial end of the cuts of thefirst set.

[0088] In this embodiment, the two halves 202 and 302 of the sheet 2have radially oriented, angularly equally spaced cuts. The angular pitchof the individual cut positioning radiuses is of 20°. Such angulardistance is maintained for all cuts, except the cuts immediatelyadjacent to the line that divides the sheets into the two halves 202 and302. In fact, the cuts of the half 302 are all rotated by 10° forwardwith respect to the last adjacent cut 102 on the first sheet half 202.The resulting sheet is as shown in FIGS. 13 and 14, i.e. by using thesame sheet pack forming arrangements, an angular offset is providedbetween the radial cuts of the underlying sheet and the sheet layingover the latter in an overturned position. FIG. 15 shows the position ofthe combined cuts of the two sheets. By overturning the overlying sheet,an interleaved arrangement of the radial cuts of the two adjacent sheetsis obtained. The cuts of one sheet are disposed in a centered positionwith respect to two cuts of the other sheet, with no coincidence of cutsbeing generated in the pack of sheets.

[0089] Here, the advantage as regards construction consists in that thesheet is not required to be rotated, but simply has to be overturnedbefore laying it over the top sheet of the pack. In order to cause thecuts to be offset, no mutual rotation of the sheets is required, whichwould be only possible by using circular sheets.

[0090] Furthermore, by properly positioning the centering pins, the cutsare properly offset by simply alternately overturning the sheets as theyare placed on the pack being formed, and fitting them onto the centeringpins.

[0091] Regarding all the embodiments as shown in the figures, relativelysimple arrangement patterns have been proposed, for the sake ofsimplicity, however the principle of this invention includes any type ofarrangement pattern.

[0092]FIGS. 16 and 17 show another advantageous feature, that may beprovided in combination with any sheet described in the previousembodiments of FIGS. 1 to 15. In FIGS. 16 to 18, cut arrangementpatterns have been omitted from the sheets.

[0093]FIG. 16 shows the principle of this additional feature in thesimplest embodiment. According to the invention, each sheet 2, 2′ may becomposed of two parts P1, P2 separated along a predetermined partingline, which parting line is in such position and has such an orientationrelative to the peripheral shape of the sheets that, as the sheets arestacked in alternately overturned positions, as described in greaterdetail above, the parting lines between the two sheet parts are disposedin non coincident positions and/or in such a manner as to avoid orminimize intersections.

[0094] Given a sheet as described above with reference to FIGS. 1 to 15,wherein the cut arrangement pattern is differentiated for each sheethalf, the parting line is selected in such position and with such anorientation as to coincide neither with the center sheet overturningaxis and/or with a center axis parallel to an overturning axis, nor witha cut 102, 102′ positioning axis.

[0095]FIG. 16 shows two superimposed sheets 2 and 2′, one in a firstposition and the other in a second position, the latter corresponding toan overturned superimposed position of said second sheet 2′ on theformer 2. The parting line 5 is eccentric to the center overturningaxis, which is outlined by a dashed line. As is apparent from the above,by overturning the sheet, the eccentric parting line 5 of the sheet 2 isnot coincident with the parting line 5 of the overturned sheet 2′,whereby the pack of sheets is not divided, along a perpendicular planeof the pack of sheets, into two packs of sheets, continuously extendingthrough the pack.

[0096] Obviously, the simplest embodiment of FIG. 16 may be improved byproviding sheets separated into three or more parts, either in onedirection or in both directions.

[0097]FIG. 17 shows an example of arrangement of parting lines in asheet composed of four parts, by providing a complex and branchedparting line 5. In this case, a minimum number of intersections cannotbe avoided, particularly the point denoted C. The dotted line, denoted5′, outlines the combined parting line obtained in the overturned andsuperimposed sheet, whereas the dashed lines outline the two centerorthogonal axes, referred to the plan view of the whole sheet. The wholesheet 2 is composed in this case by four smaller parts P1, P2, P3, P4.The above mentioned intersections, and in this specific case the pointC, are such as to generate a point-like slot in the pack of sheets,which continuously extends across the whole pack. This drawback does notaffect significantly the suppression or drastic reduction of undesiredcurrents produced in the pole pieces.

[0098] The advantage of this embodiment consists in that it allows todesign relatively inexpensive magnetic structures which are notsubjected to the size restrictions imposed by metal sheet manufacturers.It shall be considered that, while these construction parts arerelatively large and not complex from the mechanical point of view, theuse thereof in Nuclear Magnetic Resonance Imaging apparatuses requiressize tolerances that are unusual for the field of metal production.Also, the material used for the sheets is often a special alloy that isnever available in the large sizes required by Magnetic ResonanceImaging apparatus manufacturers. The provision of sheets composed of atleast two or more parts also allows to use special materials, withoutforcing the Magnetic Resonance Imaging apparatus manufacturer to requirespecial, high cost productions.

[0099] Finally, FIG. 18 shows an improvement of the principle as shownin FIGS. 16 and 17. Here, the parting line 5 is not straight but mayhave either a rounded or a polygonal profile. Particularly, it has atleast two partial segments which form a certain angle. This provides acertain simplified mutual centering and alignment action between the twosheet parts separated by said line 5. Here again, the sheets may becomposed of three or more parts.

[0100] The non-straight parting line may have several differentprofiles, e.g. a sinusoidal profile or a toothed profile, with square,triangular, rounded, circular teeth, etc. still considering that theenvelope of the parting line shall always preferably occupy a surfaceband not coincident with the center overturning axis or with the centeraxis parallel to the overturning axis, but possibly intersecting thecuts or partly coincident with a cut. FIGS. 21 to 25 show a few nonlimiting examples of different possible embodiments of the parting line.The dashed line is the center axis of the sheet, parallel to orcoincident with the overturning axis of the sheets for forming the packaccording to this invention.

[0101] In FIGS. 26 to 28 a further embodiment of the sheets for themagnet structure according to the invention are shown. According to thisembodiment, two kind of sheets provided with a pattern of passingthrough cuts are provided. The sheets has been given the same numeral asin the FIGS. 1 to 4, namely 2, 2′. In this example each sheet 2, 2′ isprovided with an identical pattern of cuts 102. However in a differentmanner as in the examples of FIGS. 1 to 4, the sheets are not providedfor being superimposed by turning them upside and downside for eachlayer but two sheets 2, 2′ are provided having the identical andcongruent peripheral shape and having an identical pattern of cuts 102which patter has a different position relatively to the sheet shape.According to this embodiment the pattern of cuts on sheet 2′ is providedon the second sheets 2′ in a shifted way with respect to the position ofthe same pattern of cuts provided on the first sheet 2, according to twodirections which are transverse one with respect to the other,particularly orthogonal one to the other and which are parallel to theplane defined by the sheet. Thus superimposing sheet 2′ on sheet 2, thecuts of the pattern of the two sheets will not coincide and will beprovided laterally staggered relatively to one another as illustrated inFIG. 28. Forming a pack of sheets by alternatively superimposing sheetsof the first kind 2 on sheets of the second kind 2′ the pack of sheetswill never show coincident cuts 102, 102′ of two adjacent sheets.

[0102] Although a circular peripheral shape of the sheets is notmandatory, if the shape of the sheets do not have a rotational symmetry,the alignment of the sheets in the pack can be made by referring to theshape of the peripheral borders. When the symmetry of the of the sheetsis a rotational one it is preferred to provide both first and secondkind of sheets 2, 2′ with identical pattern of centering through holes502 which pattern have the same position for both kinds of sheets 2, 2′and fall congruent when the sheets are superimposed. This helps incorrectly aligning and orienting the two kind of sheets in order toavoid crossing of the cuts 102, 102′ of the two kinds of sheets.

[0103] Having chosen to shift the pattern of cut on one sheet 2′ ascompared to the same pattern of cuts on the other kind of sheets 2 alongtwo orthogonal directions allows to provide a pattern of cuts which canshow cuts oriented according to these two directions and cuts which areoriented at 45° or 135° or 225° or 315° with respect to the said twoorthogonal directions.

[0104] Many kinds of patterns are possible which will lead to the resultof efficiently limiting or suppressing eddy currents induced in the polepieces of the magnet structure.

[0105] The present example shows a particular pattern which is formed byproviding concentric squares paths having a constant length increasefrom one square to the other inscribed within a circular peripheral edgeof the sheets. The concentric squares have a common center which isdisplaced according to two directions relatively to the center of thecircular shape of a first sheet kind 2 while the center of theconcentric squares are displaced in the same two directions but on theopposite side of the centre of the circular sheet on the second kind ofsheets 2′. Cuts are provided along the sides of the concentric squares,the cuts being discontinuous in order to form material connectionsbetween the surface of the sheet enclosed by the cuts on a square pathand the material of the sheet outside the said square path. Thussuperimposing the two sheets each square path on a first kind 2 of sheetand thus the cuts provided along the said path will be placed laterallysifted relatively to the same square path and the cuts provided there onof the second kind of sheets. At the annular surfaces of the sheetsprovided between two following square path along which the cuts areprovided further L shaped cuts are provided which are oriented accordingto the alignment of a further concentric square path for the cuts whichis enclosed by the innermost one and has a 90° rotated orientation.Further smaller L cuts are also provided to fill in a regular manner thelast zones of the sheets which are still free of cuts.

[0106] Although the above described embodiment provides two differentkind of sheets the embodiment could be also designed in order to beobtained by only one kind of sheet having a different positioning of thepattern of cuts onto different halves, whereby the pattern of cuts isidentical for both halves of the sheet but the one on a first half isshifted along two orthogonal directions on the other half with respectof the position of the pattern of cut on the first half.

[0107] All the other features disclosed for the previous embodiments andwhich can apply also to the present embodiment has to be considered asprovided in any combination or subcombination with this last embodiment.

[0108] Providing different kinds of sheets with shifted pattern of cutsaccording to the above described example of FIGS. 26 to 28 gives theopportunity to increase the number of kinds of sheets. Thus for examplea sequence of N different sheets with N integer can be provided each onehaving an identical pattern of cuts which for each one of the N sheetshas been displaced for a given distance along the same two directionsrelatively to the preceding sheet in the sequence of sheets.

[0109] N can be a rational fraction of the total number of sheets neededfor forming the pack of sheets thus an alternate superimposing of the Ndifferent kind of sheets in the pack corresponding to a recursivesuperimposing of the sequence of sheets is needed. N can be also equalto the total number of sheets provided in the pack of sheets so that nonrepetition of the sequence of the N different kind of sheets is neededfor forming the pack of sheets.

[0110] Although only preferred embodiments are specifically illustratedand described herein, it will be appreciated that many modifications andvariations of the present invention are possible in light of the aboveteachings and within the purview of the appended claims withoutdeparting from the spirit and intended scope of the invention.

What is claimed is:
 1. A magnet structure for Nuclear Magnetic Resonanceimaging apparatus, comprising: at least two opposing magnetic polepieces located at a certain distance from each other and which delimitan imaging region; wherein the pole pieces are formed by at least onemassive layer of a magnetically permeable material, and at least onelayer of magnetically permeable material comprising a pack ofsuperimposed sheets or foils, electrically insulated from each other,each sheet having cuts arranged over the surface of the sheet inpositions that are at least partly non coincident with the cuts of atleast one, or both adjacent sheets, and wherein each of the magneticallypermeable sheets or foils comprises: a first face; and a second face;and wherein the cuts arranged over the surface of the sheet or foil havea width and length and are so arranged on each sheet, that the cuts of asheet or foil are offset and not coincident with respect to the cuts ofan adjacent sheet or foil, when the adjacent sheet is laid over theprevious sheet in an overturned position, with the first face turnedtoward the first face of the first sheet or with the second face of saidadjacent sheet turned toward the second face of the first sheet.
 2. Amagnet structure as claimed in claim 1, wherein each sheet is dividedinto two halves along an axis parallel to or coincident with a sheetoverturning axis, about which each successive sheet is overturned by180° relative to the adjacent preceding sheet of the layer of sheets ofeach pole piece, there being provided an identical pattern of cuts forall sheets, which pattern differs in the arrangement, orientation,length, and/or width of the cuts in the two halves of each sheet suchthat, when two sheets are superimposed in a mutually overturned orreversed condition, with the first faces or the second faces of said twosuperposed sheets in contact with each other, the cuts of a sheet aredisposed in offset positions with respect to the cuts of the overlyingsheet in both halves of said adjacent sheets.
 3. A magnet structure asclaimed in claim 1, wherein the patterns of cuts on a sheet have arepeated, orderly, and/or recurring geometric cut arrangement, the cutsof the two halves of the sheet being formed according to the samearrangement pattern which, in the second half of the sheet, is offset toa certain extent in one or two directions subtending the plane formed bythe sheet and/or angularly also in a possible direction of rotation orcombination of said displacements, with respect to the periphery ofsecond sheet half, relative to the position in the arrangement patternof the first half of the sheet, such that the cuts of two superposedsheets, adhering by their first faces and their second faces, coincidewith cut-free portions and/or have a small number of intersection pointsbetween the cuts of the two superposed sheets.
 4. A magnet structure asclaimed in claim 3, wherein when two adhering sheets are superimposed,one of the two sheets is overturned with respect to the other sheet,such that the first half and the second half of a sheet lie over thesecond half and the first half respectively of the other sheet and thecuts of said first and second halves of the one sheet are disposedcoincident with the cut-free portions of said second and first halves ofthe underlying sheet and vice versa.
 5. A magnet structure as claimed inclaim 1, wherein the cuts are disposed along parallel axes which form aset of parallel axes, the sets of axes on the first and on the secondhalf of the sheet being oriented parallel to each other and to anoverturning axis, and there being provided a distance of the first axisof each set of axes from the center axis of the sheet, which is parallelto or coincident with the overturning axis, said distance beingdifferent for the sets of axes on the first half and on the second halfof the sheet respectively.
 6. A magnet structure as claimed in claim 5,wherein the cuts are continuous along the corresponding positioning axisof the set of parallel positioning axes.
 7. A magnet structure asclaimed in claim 5, wherein the cuts are discontinuous along thecorresponding positioning axis of the set of parallel positioning axesand form whole regions or bridges of sheet material.
 8. A magnetstructure as claimed in claim 1, wherein the sets of cut positioningaxes on the two halves of the sheet have an inclined, symmetricallydivergent or convergent orientation, for the first and the second halvesof the sheet with respect to the center axis of the sheet, which isparallel to or coincident with the sheet overturning axis, and whereinthe intersection points of the set of parallel positioning axes on thefirst half of the sheet with said center axis being provided inintermediate positions between the intersection points of the set ofparallel positioning axes of the second sheet half of the sheet.
 9. Amagnet structure as claimed in claim 8, wherein the intersection pointsof the two sets of parallel cut positioning axis on the first and secondhalves of the sheet are interleaved and equally spaced along the centeraxis, which is parallel to or coincident with the overturning axis. 10.A magnet structure as claimed in claim 1, wherein the sets of cutpositioning axes on the two halves of the sheet have an inclinedorientation with respect to the center axis of the sheet, which isparallel to or coincident with the sheet overturning axis, theintersection points between the set of parallel positioning axes on thefirst half of the sheet and said center axis being situated inintermediate positions with respect to the intersection points betweenthe positioning axes of the set of parallel positioning axis of thesecond half of the sheet and said center axis, and each cut along eachpositioning axis being discontinuous and forming an unbroken portion ofthe sheet, the succession of the unbroken sheet portions and of the cutparts along the positioning axes being inverted from the first half tothe second half of the sheet, whereas the pitch of the cuts and unbrokenportions along each positioning axis is such that, when the first halfof the sheet is overturned against the second half of the sheet, thecuts of the first half of the sheet intersect along each positioningaxis the cuts of the second half of the sheet at unbroken portions, andvice versa.
 11. A magnet structure as claimed in claim 1, wherein oneach sheet half, the cuts are arranged along two intersecting sets ofparallel positioning axes, an unbroken portion of the sheet or a bridgeof material being provided at each intersection point between each axisof one set of axes, and each axis of the other set of axes, and the twosets of intersecting parallel positioning axes being offset in thesecond half of the sheet relative to the corresponding set of the firsthalf of the sheet, such that, when the second half of the sheet isoverturned on the first half of the sheet about a center axis parallelto the overturning axis or coincident therewith, the unbrokenintersection portions of the cuts along intersecting sets of parallelpositioning axis of one half of the sheet fall within portions of thesheet of the other half of the sheet, which are delimited by the cuts.12. A magnet structure as claimed in claim 11, wherein the intersectingsets of positioning axes have axes inclined with respect to theoverturning axis, in the same orientation for the two sheet halves orwith different, symmetrical orientations with respect to a center axisparallel to and/or coincident with the overturning axis.
 13. A structureas claimed in claim 11, wherein one set of parallel cut positioning axeson both sheet halves is parallel to the overturning axis.
 14. Astructure as claimed in claim 11, wherein one set of parallel cutpositioning axes on both sheet halves is perpendicular to theoverturning axis.
 15. A structure as claimed in one or more of thepreceding claims 11, wherein on at least one half of the sheet, the twointersecting sets of positioning axes are disposed in such a manner thatthe unbroken sheet portions between cuts along the axes of a first setare situated in intermediate positions between two positioning axes ofthe second set, whereas the cuts of the first set intersect the axes ofthe second set coincident with the unbroken portions along the axes ofsaid second set.
 16. A structure as claimed in claim 1, wherein the cutsare disposed along positioning axes in radial arrangements from adefined center of the sheet, an identical angular distance beingprovided between individual radial lines along which cuts are provided,whereas a portion of said radial arrangement of cuts being provided oneach of the two sheet halves, the portion of radial cuts of the secondsheet half being rotated with respect to the portion of the radial cutsof the first sheet half, with reference to said center, to such anextent that when the second half of the sheet is overturned on the firsthalf of the sheet, the cuts of one of the two halves are in intermediatepositions between the cuts of the other half.
 17. A magnet structure asclaimed in claim 16, wherein the center of the radial arrangement ofcuts is the same for both halves of the sheet and is coincident with acenter axis, parallel to or coincident with the overturning axis and/orthe geometric center of the sheet.
 18. A magnet structure as claimed inclaim 16, wherein the radial cuts on the sheet are combined with aperipheral ring of cuts, which is interleaved with the former, the cutsof the peripheral ring being also disposed on the first and secondhalves of the sheet and the cuts of the second sheet half beingangularly shifted with respect to those of the first sheet half, likethe radial cuts.
 19. A magnet structure as claimed in claim 16, whereineach sheet is composed of at least two adjacent sheet parts, separatedby a parting line, said parting line being provided in eccentricposition and/or orientation with respect to the center axis parallel toor coincident with the overturning axis, and anyway in such positionand/or orientation that the parting lines between the parts of twosuperimposed sheets do not coincide, and intersection pointstherebetween are avoided or minimized.
 20. A magnet structure as claimedin claim 19, wherein the parting line between the two or more parts ofthe sheet extends in a cut-free portion, so that said parting line doesnot intersect any cut on the sheet and/or possibly any cut on one or twoadjacent sheets.
 21. A magnet structure as claimed in claim 19, whereinthe parting line is a polygonal line, a toothed line with square,triangular or trapezoidal teeth, a curved line, or a combination of saidlines.
 22. A magnet structure as claimed in claim 19, wherein theparting line is a polygonal line having transverse branches and dividesa sheet into four separate parts.
 23. A magnet structure as claimed inclaim 19, wherein the sheets and/or the sheet parts have holes atpredetermined coincident positions for engagement on alignment andcentering pins during the assembly of the pack of sheets.
 24. A methodfor making a magnet structure for Nuclear Magnetic Resonance imagingapparatus, which magnet structure has at least two opposing magneticpole pieces, which are located at a certain distance from each other anddelimit an imaging region, which pole pieces are formed by at least onemassive layer of a magnetically permeable material, and at least onelayer of magnetically permeable material comprising a pack ofsuperimposed sheets or foils, electrically insulated from each other,each of which sheets has an upper face and a rear face with respect tothe massive layer and each of which sheets has cuts arranged over thesurface of the sheet in positions that are at least partly noncoincident with the cuts of at least one, or both adjacent sheets,wherein the cuts are formed on each sheet with such a pattern of cutsthat the cuts of an overlying sheet or foil are disposed in offset andnot coincident positions with respect to the cuts of the underlyingsheet, when said overlying sheet is laid over the preceding sheet in areversed position, i.e. with the front face overturned against thepreceding sheet, the pack of sheets being formed by the alternatedisposition of said sheets in a normal position, with the upper faceturned toward the massive layer and the sheets in the overturnedposition.
 25. A method as claimed in claim 24, further comprising thestep of dividing the surface of the sheets into two halves, cuts beingformed on one sheet half in a first pattern, and cuts being formed onthe second half of the sheet in a pattern related to the pattern of cutson the first sheet, in such a manner that when the second half of thesheet is overturned against the first half of the sheet, the cuts of thetwo halves of the sheet are disposed in non coincident positions and/orsuch as to avoid or minimize intersections, the pack of sheets beingformed by identical sheets all having the same patterns of cuts on thecorresponding first and second halves of the sheets and said sheetsbeing alternately superimposed with a first face turned toward themassive layer and with said first face turned in the direction oppositethe massive layer of the pole piece.
 26. A method for making a magnetstructure for Nuclear Magnetic Resonance imaging apparatus, which magnetstructure has at least two opposing magnetic pole pieces, which arelocated at a certain distance from each other and delimit an imagingregion, which pole pieces are formed by at least one massive layer of amagnetically permeable material, and at least one layer of magneticallypermeable material comprising a pack of superimposed sheets or foils,electrically insulated from each other, each of which sheets has anupper face and a rear face with respect to the massive layer and each ofwhich sheets has cuts arranged over the surface of the sheet inpositions that are at least partly non coincident with the cuts of atleast one, or both adjacent sheets, wherein the cuts are formed on eachsheet with such a pattern of cuts that the cuts of an overlying sheet orfoil are disposed in offset and not coincident positions with respect tothe cuts of the underlying sheet, when said overlying sheet is laid overthe preceding sheet in a reversed position, i.e. with the front faceoverturned against the preceding sheet, the pack of sheets being formedby the alternate disposition of said sheets in a normal position, withthe upper face turned toward the massive layer and the sheets in theoverturned position, further comprising the step of forming a pattern ofcuts as claimed in claim
 3. 27. A method as claimed in claim 24, furthercomprising the step of interposing a film between two adjacent sheetsand/or of coating each sheet with an insulating and/or adhesivematerial, or of coating at least one face of each sheet with a layer ofan insulating and/or adhesive material before superimposing the sheetsof the pack of sheets being formed.
 28. A method as claimed in claim 27,wherein the pack of sheets with interposed insulating and/or adhesivelayers, is pressed under heat or at ambient temperature.
 29. A method asclaimed in claim 27, wherein a thermosetting adhesive film is interposedbetween the sheets or at least one face of the sheets is coated withsaid thermosetting adhesive, the pack being hot and vacuum pressed. 30.A method as claimed in claim 29, wherein the layer of thermosettingadhesive consists of a compound used for making printed circuit boards,known as vetronite or preprag.
 31. A method as claimed in claim 24,wherein each sheet is composed of at least two separate sheet parts, theparting line between said at least two sheet parts being provided ineccentric position and/or orientation with respect to the center axisparallel to or coincident with the overturning axis, and anyway in suchposition and/or orientation that the parting lines between the parts oftwo superimposed sheets do not coincide, and intersection pointstherebetween are avoided or minimized.
 32. A method as claimed in claim24, wherein each sheet is formed by three or more sheet parts, theparting line being a polygonal line with lateral branches, which extendsso that the parting lines of two superposed sheets, one whereof isoverturned, are not superposed and do not intersect, or intersect in aminimized number of points.
 33. A magnet structure as claimed in claim24, wherein the parting lines are at least partly a polygonal line, atoothed line with square, triangular or trapezoidal teeth, a curvedline, or a combination of said lines.
 34. A method as claimed in claim24, wherein the parting lines are situated in regions without cuts. 35.A magnet structure for Nuclear Magnetic Resonance imaging apparatus,which magnet structure has at least two opposing magnetic pole pieces,which are located at a certain distance from each other and delimit animaging region, which pole pieces are formed by at least one massivelayer of a magnetically permeable material, and at least one layer ofmagnetically permeable material comprising a pack of superimposed sheetsor foils, electrically insulated from each other, each of which sheetshas cuts arranged over the surface of the sheet in positions that are atleast partly non coincident with the cuts of at least one, or bothadjacent sheets, wherein at least two different kinds of magneticallypermeable sheets or foils are provided which have passing cutsdistributes on their surface area according to an identical pattern ofcuts and the pattern of cuts on a first kind of sheets is provided onthe said first kind of sheets in a shifted position with respect to theposition of the same pattern of cuts provided on the second kind ofsheets, according to two directions which are transverse one withrespect to the other, particularly orthogonal one to the other and whichare parallel to the plane defined by the sheets in such a way that bysuperimposing a sheet of the first kind on a sheet of the second kind,the cuts of the pattern of the two kind of sheets will not coincide. 36.A magnet structure according to claim 35, wherein a sequence of Ndifferent kind sheets with N being integer is provided each one kind ofsheets having an identical pattern of cuts which for each one kind ofthe N kind of sheets is displaced for a given distance along the sametwo directions relatively to the preceding kind of sheet in the sequenceof the N kind of sheets.
 37. A magnet structure according to claim 36,wherein the number N of different kinds of sheets is a rational fractionof the total number of sheets in a pack of sheets and the pack of sheetsis formed by an alternate superimposing of the N different kind ofsheets in the pack corresponding to a recursive superimposing of thesequence of sheets.
 38. A magnet structure according to claim 36,wherein the number of the different kind of sheets N is equal to thetotal number of sheets provided in the pack of sheets so that nonrepetition of the sequence of the N different kind of sheets is neededfor forming the pack of sheets.
 39. A magnet structure according toclaim 1, wherein the sheets have non rotational symmetry.
 40. A magnetstructure according to claim 1, wherein the sheets have rotationalsymmetry.
 41. A magnet structure according to claim 1, wherein thesheets are provided with one, two or more centering through holes whichare distributed over the surface area of the sheets according to apattern which is identical for each sheet and which patter has the sameposition on each sheet in such a way that superimposing the sheets forforming a pack of sheets the centering holes are coincident.
 42. Amagnet structure according to claim 1, wherein the sheets have identicalshape and dimensions.
 43. A magnet structure according to claim 1,wherein the pattern of cuts is formed by rectangular concentric idealpaths along which paths several cuts are provided at a certain distanceone from the other for forming material bridges between the sheet zonesinside a rectangular path and outside the said rectangular path.
 44. Amagnet structure according to claim 1 wherein the pattern of cutsprovides cuts having orientations parallel to both the two directions ofdisplacement of the pattern of cuts between two different kinds ofsheets and also cuts oriented along paths having at an angle of 45°,135°, 225°, or 315° with respect to the said two orthogonal directionsof displacement.
 45. A method for making a magnet structure for NuclearMagnetic Resonance imaging apparatus, which magnet structure has atleast two opposing magnetic pole pieces, which are located at a certaindistance from each other and delimit an imaging region, which polepieces are formed by at least one massive layer of a magneticallypermeable material, and at least one layer of magnetically permeablematerial comprising a pack of superimposed sheets or foils, electricallyinsulated from each other, each of which sheets has cuts arranged overthe surface of the sheet in positions that are at least partly noncoincident with the cuts of at least one, or both adjacent sheets, themethod comprising: providing at least two different kinds ofmagnetically permeable sheets or foils providing the two kinds of sheetswith passing cuts distributed on their surface area according to anidentical pattern of cuts and the said pattern of cuts on a first kindof sheets being in a shifted position with respect to the position ofthe same pattern of cuts provided on the second kind of sheets,according to two directions which are transverse one with respect to theother, particularly orthogonal one to the other and which are parallelto the plane defined by the sheets in such a way that by superimposing asheet of the first kind on a sheet of the second kind, the cuts of thepattern of the two kind of sheets will not coincide.
 46. A methodaccording to claim 45, wherein N different kind of sheets are providedthe N kind of sheets being part of a sequence of different kind ofsheets obtained by stepwise shifting an identical pattern of cuts of onekind of sheet of the sequence relatively to the previous kind of sheetof the sequence and providing the said kind of sheet with discontinuouscuts along the said pattern.
 47. A method for making a magnet structurefor Nuclear Magnetic Resonance imaging apparatus, which magnet structurehas at least two opposing magnetic pole pieces, which are located at acertain distance from each other and delimit an imaging region, whichpole pieces are formed by at least one massive layer of a magneticallypermeable material, and at least one layer of magnetically permeablematerial comprising a pack of superimposed sheets or foils, electricallyinsulated from each other, each of which sheets has cuts arranged overthe surface of the sheet in positions that are at least partly noncoincident with the cuts of at least one, or both adjacent sheets, themethod comprising: providing at least two different kinds ofmagnetically permeable sheets or foils providing the two kinds of sheetswith passing cuts distributed on their surface area according to anidentical pattern of cuts and the said pattern of cuts on a first kindof sheets being in a shifted position with respect to the position ofthe same pattern of cuts provided on the second kind of sheets,according to two directions which are transverse one with respect to theother, particularly orthogonal one to the other and which are parallelto the plane defined by the sheets in such a way that by superimposing asheet of the first kind on a sheet of the second kind, the cuts of thepattern of the two kind of sheets will not coincide, wherein it providesthe steps of claim 27.